Lithium ion secondary batteries have high energy density and high capacity, and thus are widely used as driving power sources for mobile data terminals and the like. In recent years, the use of lithium ion secondary batteries for industrial applications, for example, installation in electric and hybrid automobiles that require large capacity, is also increasing, and studies for increasing the capacity and performance even more are conducted. One of such studies attempts to increase the charge and discharge capacity using silicon or tin, which can occlude a large amount of lithium per unit volume, or an alloy containing silicon or tin, as an active material for a negative electrode.
However, when an active material having a large charge and discharge capacity, such as silicon, tin, or an alloy containing silicon or tin, for example, is used, the active material significantly changes its volume in accordance with charging and discharging. For this reason, if polyvinylidene fluoride or a rubber resin, which are used for conventional electrodes, is used as a binding agent (binder), the problem arises in that due to breakage of an active material layer or the occurrence of detachment at an interface between a current collector and the active material layer, a current collecting structure of the electrode may be broken, and thus the cycle characteristics of the battery may easily deteriorate.
Thus, there is a demand for a binder for an electrode, the binder being unlikely to cause breakage or detachment of an electrode even when a significant change in volume occurs and having high toughness in a battery environment. Patent Literature 1 discloses that when a polyimide resin is used as a binding agent for a negative electrode of a lithium secondary battery, the battery capacity hardly decreases even after repeated charge-discharge cycles, and thus a long cycle life is achieved. According to this document, the electrode is manufactured by performing heat treatment at 350° C. for 2 hours (see Examples 1 and 2).
Patent Literature 2 discloses a binder resin composition for an electrode, the binder resin composition including a specific polyamic acid and a solvent and exhibiting a low degree of swelling in an electrolyte solution and excellent toughness (high breaking elongation and breaking energy). Moreover, it is disclosed that during manufacturing of the electrode, heat treatment at a relatively high temperature is required so that an imidization reaction proceeds to a sufficient extent.
Patent Literature 3 discloses a resin composition for an electrode of a lithium ion secondary battery, the resin composition containing a polyimide resin having a carboxyl group and an epoxy resin.
These polyamic acid solution compositions require the use of organic solvents, and heat treatment at high temperatures. Therefore, these polyamic acid solution compositions cannot be said environmentally preferable, and there have been cases where their applications are limited. In view of this, it has been proposed to use water-soluble polyimide precursors. For example, Patent Literature 4 discloses a method for manufacturing an electrode using an aqueous polyimide precursor solution composition obtained by dissolving a polyamic acid in an aqueous solvent together with an imidazole in an amount of 1.6 moles or more per mole of a tetracarboxylic acid component of the polyamic acid, the imidazole having two or more alkyl groups as substituents.
Moreover, Non-Patent Literature 1 states that a lower degree of swelling of a binder resin for an electrode in an electrolyte solution leads to a higher discharge capacity retention after charge-discharge cycles and is thus preferred.